An elated John Thompson has his first Golden Slipper runner, with debutant Mamaragan securing a start after upstaging his more experienced rivals to win the Skyline Stakes at Randwick.

The colt was ridden brilliantly by Blake Shinn, who flew in from Hong Kong to partner Thompson's three stakes runners at Randwick on Saturday, replacing the trainer's regular go-to jockey Nash Rawiller who is suspended.

"I'm not a trainer that aims horses up at the Slipper but he has just been a natural from day one and I'm rapt to have one in it," Thompson said.

"The sky's the limit. You saw today he was very raw in front. He didn't know what to do but he will improve a lot from the run."

Thompson had been so impressed by the youngster at home he was the only two-year-old kept in the Golden Slipper mix.

"He was the only one I accepted with," Thompson said.

"I'm sure he will do well, he was a good-doing colt with a great attitude and it's exciting."

Shinn camped Mamaragan ($19) outside the leader and out of trouble, with the youngster kicking strongly to win by 1-1/4 lengths over Osamu ($19). Aim ($4.80) closed late another three-quarters of a length away.

It was an emotional victory for Shinn, who left Sydney for Hong Kong in July last year and has had a long association with the stable.

He says Mamaragan deserves an historic shot at the Slipper when the colt will be vying to become the most inexperienced winner of the race, a record held by Toy Show, who claimed it at her third start in 1975.

"He is raw, he is untapped, why not? Why not have a go?" Shinn said.

"You're only a two-year-old once. This is a lead-up to the Slipper and he's done it in fine style."

Prague started a $2.60 favourite and never threatened, finishing sixth.

Co-trainer Ciaron Maher was perplexed by the performance and felt the horse might have choked down when he was restrained mid-race.

However, he confirmed he planned to press on to the Slipper with Prague, provided the previously unbeaten colt came through the run well.

"He seemed fine when he came back but maybe he choked or something when he was restrained," Maher said.

"He is still improving and we will see how he is in the next couple of weeks but the Slipper has always been the plan for him."

Mamaragan has firmed into $26 for the Slipper, in from his opening quote of $101.